Jack-up rigs are widely used in offshore environments where a stable vessel is required to perform an operation such as maintenance of an offshore structure.
A jack-up rig is a floating vessel which can be sailed into position above or adjacent to an offshore structure. Once in position, the jack-up rig's legs are lowered into engagement with the seabed. Once securely standing on the seabed, the hull can be raised out of the water and secured with respect to the legs of the vessel.
The relative movement between the legs and the hull of the jack-up rig is achieved by a rack and pinion type arrangement. Each leg defines a rack gear which engages with a pinion gear attached to the hull. Each pinion gear is rotated with respect to its associated rack creating relative movement, in the first instance to lower the legs to the seabed and in the second instance to raise the hull out of the water. Once these manoeuvres are completed, the pinion gear is held stationary thereby preventing further movements of the legs with respect to the hull.
This arrangement has drawbacks, for example, the wear on each of the pinion gears and the associated reduction gearbox can be considerable requiring regular replacement of the pinion gears and overhaul or replacement of the gearbox.
Additionally, the rack and pinion arrangement requires the pinion gears to be locked in position to prevent relative movement between the hull and the legs. Failure of this engaging relationship can result in catastrophic consequences.
Other systems of moving and engaging the hull with respect to the legs using linear rather than rotational engagement are known, however these systems encounter time-consuming difficulties associated with aligning the respective components when the components engage and disengage.